1. Field of the Invention
The present invention relates to a process for removing methacrylic acid from liquid phase comprising acrylic acid as a main constituent and target product, and methacrylic acid as a secondary component.
2. Description of the Background
Acrylic acid is an important monomer which finds use as such or in the form of its alkyl esters for obtaining, for example, polymers suitable as adhesives or water-superabsorbent polymers (cf., for example, WO 02/055469 and WO 03/078378).
Acrylic acid is prepared on the industrial scale worldwide virtually exclusively by the (generally two-stage) heterogeneously catalyzed partial oxidation process of propylene (cf., for example, EP-A 990 636, U.S. Pat. No. 5,198,578, EP-A 10 15 410, EP-A 14 84 303, EP-A 14 84 308, EP-A 14 84 309 and US-A 2004/0242826). The starting propylene used is propylene of comparatively high purity (cf. DE-A 101 31 297). It is relatively inconvenient and costly to obtain such pure crude propylene. It normally originates from crude paraffinic hydrocarbons and generally includes various purification stages in order to isolate the propylene formed in highly pure form (cf. DE-A 35 21 458). These purification stages generally comprise separations from olefins other than propylene and from other secondary products other than propylene, including the secondary components already comprised in the crude paraffinic hydrocarbon.
Of particular significance in this context is the separation of propylene from its companion propane. Owing to the physical similarity of the two compounds, this removal in particular is capital- and energy-intensive. Since the predominant amount of the thus obtained crude propylene is used in large amounts for subsequent polymerizations (for example to prepare polypropylene), (where the high purity described is indispensable) and experiences a high addition of value, the aforementioned removals are customary in conjunction with refinery crackers and steamcrackers in spite of the associated cost and inconvenience, and form the state of the art in industry. The proportion of these crude propylenes flowing into the partial oxidation to acrylic acid is of rather minor importance compared to the demand for polypropylene, and is a secondary demand stream at raw material prices which are still acceptable.
It is characteristic of the preparation of acrylic acid by catalytic partial oxidation in the gas phase of such comparatively pure propylene that the acrylic acid, in spite of the available purity of the raw material, is not obtained as such, but rather, especially owing to parallel side reactions, as a constituent of a product gas mixture from which it subsequently has to be removed.
This product gas mixture generally also comprises reactants which have not been converted fully, if appropriate intermediates which have not been converted fully and, for reasons of improved heat transfer and to ensure nonexplosive behavior, additionally used inert diluent gas.
In this document, an inert diluent gas shall be understood to be a reaction gas constituent which behaves inertly under the conditions of the relevant reaction and, each inert reaction gas constituent viewed alone, remains chemically unchanged to an extent of more than 95 mol %, preferably to an extent of more than 97 mol %, or 98 mol % or 99 mol %.
A common feature of substantially all separation processes known in this regard is that, if appropriate after direct and/or indirect cooling of the aforementioned product gas mixture, acrylic acid comprised in the product gas mixture is transferred in a basic removal step into the condensed (especially liquid) phase.
This may be effected, for example, by absorption into a suitable solvent (e.g. water, high-boiling organic solvents, aqueous solutions) and/or by partial or substantially full condensation (e.g. fractional condensation) (on this subject, see the documents cited at the outset, and also the documents EP-A 13 88 533, EP-A 13 88 532, DE-A 102 35 847, EP-A 79 28 67, WO 98/01415, EP-A 10 15 411, EP-A 10 15 410, WO 99/50219, WO 00/53560, WO 02/09839, DE-A 102 35 847, WO 03/041833, DE-A 102 23 058, DE-A 102 43 625, DE-A 103 36 386, EP-A 85 41 29, U.S. Pat. No. 4,317,926, DE-A 198 37 520, DE-A 196 06 877, DE-A 190 50 1325, DE-A 102 47 240, DE-A 197 40 253, EP-A 69 57 36, EP-A 98 22 87, EP-A 10 41 062, EP-A 11 71 46, DE-A 43 08 087, DE-A 43 35 172, DE-A 44 36 243, DE-A 19 924 532, DE-A 103 32 758 and DE-A 19 924 533). A removal of acrylic acid may also be undertaken as described in EP-A 98 22 87, EP-A 98 22 89, DE-A 103 36 386, DE-A 101 15 277, DE-A 196 06 877, DE-A 197 40 252, DE-A 196 27 847, EP-A 92 04 08, EP-A 10 68 174, EP-A 10 66 239, EP-A 10 66 240, WO 00/53560, WO 00/53561, DE-A 100 53 086 and EP-A 98 22 88. Favorable ways of removal are also the processes described in the documents WO 2004/063138, WO 2004/035514, DE-A 102 43 625 and DE-A 102 35 847.
The further removal of the acrylic acid from the liquid (condensed) phases which comprise the acrylic acid target product and are obtained in the basic separation described is undertaken in the processes of the known prior art, depending on the other by-products, in particular those dependent upon the specific catalysts used for the partial oxidation and other selected partial oxidation conditions, comprised in addition to acrylic acid, by a wide variety of combinations of extractive, desorptive, rectificative, azeotropically distillative and/or crystallizative processes up to the desired degree of purity of the acrylic acid. Particularly high purity demands are made on the acrylic acid when it is to be used to prepare water-superabsorbent polymers (polyacrylic acids or alkali metal salts thereof), since such polymers find use in particular in the hygiene sector, where medical standards apply. The aim of the predominant number of acrylic acid preparation processes is therefore a very economically viable route to such glacial acrylic acid suitable for superabsorbents.
A characteristic feature of the conventional acrylic acid preparation route described is that it includes by-product formation of methacrylic acid at most in amounts which are analytically undetectable or analytically insignificant, which can be attributed primarily to the high degree of purity of the crude propylene used. Thus, none of the following prior art documents even mentions methacrylic acid as a possible secondary component of acrylic acid, even though the majority of these documents comprise highly detailed secondary component analyses: WO 98/01414, WO 01/92197, EP-A 648 732, EP-A 1305097, EP-A 14 84 308, EP-A 14 84 309, US-A 2004/0242826, DE-A 103 36 386, WO 02/055469, WO 03/078378, WO 01/77056, WO 03/041833, DE-A 196 06 877, EP-A 792 867, EP-A 920 408, EP-A 10 15 411, EP-A 10 15 410, DE-A 198 38 845, WO 03/041833, WO 02/090310, DE-A 101 22 787, WO 03/041832, EP-A 10 68 174, EP-A 10 66 239, EP-A 11 63 201, EP-A 11 59 249, EP-A 11 89 861, EP-A 12 52 129, WO 01/77056, DE-A 102 35 847, DE-A 102 43 625 and WO 2004/035514. The same statement applies to the documents referred to as the particular state of the art in the aforementioned documents.
On the other hand, methacrylic acid, as would then be formed as a secondary component companion in a preparation of acrylic acid, would be a particularly unpleasant companion to acrylic acid which would not remain unmentioned in the prior art. This would be the case in particular because the tendency of methacrylic acid to free-radical polymerization, owing to the positive inductive effect of the methyl group which distinguishes it from acrylic acid, is significantly reduced in comparison to the same tendency of acrylic acid.
In other words, when acrylic acid which comprises methacrylic acid even only in traces is used to prepare water-superabsorbent free-radically polymerized polymers, it has to be assumed that the methacrylic acid is not sufficiently polymerized under the particular polymerization conditions selected and remains in the polymer formed as a vinylically unsaturated compound, which is problematic in applications in the hygiene sector. Presence of methacrylic acid can also adversely effect the polymer quality (for example molecular weight distribution, degree of crosslinking, etc.).
In the search for a more economically viable propylene source which can be used for a heterogeneously catalyzed partial oxidation to acrylic acid, it has also already been proposed to start from crude propane and convert it, in a reaction stage preceding the propylene partial oxidation, by homogeneous and/or heterogeneously catalyzed oxydehydrogenation and/or heterogeneously catalyzed dehydrogenation, partially to propylene, and to use the latter for the relevant partial oxidation without removing it in a costly and inconvenient manner from unconverted propane (cf., for example, WO 03/076370, WO 01/96271, EP-A 117 146, WO 03/011804, U.S. Pat. No. 3,161,670, DE-A 33 13 573, WO 01/96270 and the prior art referred to in these documents). According to DE-A 102 46 119 and DE-A 102 45 585, the procedure should be such that suitable separation steps ensure that the resulting starting reaction gas mixture for the propylene partial oxidation comprises a minimum level of C4 hydrocarbons as undesired impurities which impair the catalyst performance. A disadvantage of such a procedure is that the aforementioned separation operations are costly and inconvenient, and are economically in some cases prohibitive for crude propylene envisaged merely as an acrylic acid raw material, or achieve only limited separating action when they are employed more economically.
At the same time, crude propanes which comprise saturated or unsaturated C4 hydrocarbons to a significant extent are available particularly inexpensively on the market, whether they occur as poorly utilizable secondary streams on the route to the preparation of ultrapure crude propane or a costly and inconvenient C3/C4 hydrocarbon separation has been dispensed with fully in the course of their generation.
In the individual case such a favorable raw material price is then capable of economically overcompensating an accompanying reduction in the catalyst performance in a downstream heterogeneously catalyzed acrylic acid preparation by partial oxidation, or a premature requirement for a catalyst change.
A remaining disadvantage of a procedure for preparing acrylic acid which might otherwise be attractive as described is that it is accompanied, depending on catalysts used for the heterogeneously catalyzed partial oxidation of propylene afflicted with corresponding C4 hydrocarbon contents to prepare acrylic acid, by methacrylic acid secondary component formation owing to a partial oxidation of the C4 hydrocarbons (e.g. isobutene and isobutane) proceeding in parallel to the main propylene partial oxidation, with the disadvantages described (cf. DE-A 102 19 686, DE-A 33 13 573 and EP-A 297 445). The same disadvantages may accrue depending on the catalyst and reaction conditions used when acrylic acid is obtained by partial direct oxidation of propane comprising C4 hydrocarbons as impurities, as is detailed, for example, in EP-A 608 838, DE-A 198 35 247, and also the documents DE-A 102 45 585 and DE-A 102 46 119. Another possible C3 precursor which may be burdened with C4 hydrocarbons or their oxidative derivatives for a heterogeneously catalyzed preparation by partial oxidation of acrylic acid is acrolein (cf. EP-A 700 893 and EP-A 700 714).